Method for producing grain oriented silicon steel



Jan. 6, 1959 J. E. MAY 2,867,559

METHOD FOR PRODUCING GRAIN ORIENTED SILICON STEEL Filed Dec. 31, 1956 lav .90 380-- E E70- l- 5 $50.- Q4 3 S 20-- 3/ m a a .o'/ .62 .a'a .04 .05

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" "3,44%" gm 7 bymh/fmq Unite rates Patent METHOD FOR PRODUCING GRAIN ORIENTED SHJCON STEEL John E. May, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York ApplicationDecernher 31, 1956, Serial No. 631,889 I 4 Claims. c1. 14s'-111 i This invention relates to polycrystalline, magnetically soft, rolled sheet metal composed principally of an alloy of iron and silicon and, more particularly, to a process for manufacturing such materials wherein a high percentage of the grains comprising the material are each caused to have their crystal space lattices arranged in a substantially identical relationship to the plane of the sheet and to a single direction in the plane of the sheet.

The sheet materials to which my invention is related are usually referred to in the art as electrical silicon steel or, more properly, silicon-iron, composed primarily of iron alloyed with about 2.5 to 4.0 percent silicon and containing relatively minor amounts of impurities such as sulfur, manganese, phosphorus and, preferably, a very low carbon content. Such alloys crystallize in the bodycentered cubic crystallographic system at room temperature. As is well known, this refers to the symmetrical distribution or arrangement which the atoms forming the individual crystals or grains assume in such materials. In these materials the smallest prism possessing the full symmetry of the crystal is termed the unit cell and is cubic in form. This unit cube is composed of nine atoms, eight arranged at the corners of the unit cube with the remaining atom positioned at the geometric center of the cube. Each unit cell in a given grain or crystal in these materials is substantially identical in shape and orientation with every other unit cell comprising the grain.

The unit cells or body-centered unit cubes comprising these materials each have a high degree of magnetic anisotropy with respect and directions of the unit cube, and hence, each grain or crystal comprising a plurality of such unit cells exhibits a similar anisotropy. More particularly, crystals of the silicon-iron alloys to which this invention is directed are known to have their direction of easiest magnetization parallel to the unit cube edges, their next easiest direction of magnetization perpendicular to a plane passed through diagonally oppositeparallel unit cube edges and their least easiest direction of magnetization perpendicular to a plane passed through a pair of diagonally opposite atoms in a first unit cube face, the central atom and a single atom in the unit cube face which is parallel to the first face. As is well known, these crystallographic planes and identified in terms of Miller Indices, a more complete discussion of which may be found in Structure of Metals, C. S. Barrett, McGraw-Hill Book Co., New York, N. Y., 2nd edition, 1952, pp. l25 and will be referred to as, respectively, the (100) plane and the [100] direction, the (110) plane and the [110] direction, and the (111) plane and the [111] direction.

directions are conventionallyto the crystallographic planes ?atented Jan. 6, 1959 l ice 7 It has been found that these silicon-iron alloys may be fabricated by unidirectional rolling and heat treatment to form sheet or strip material composed of a plurality of. crystals or grains,'a majority of which have their atoms arranged so that their crystallographic planes have a similar or substantially identical orientation to the plane of the sheet or strip and to a single direction in said plane. This material is usually referred to as oriented or grain-oriented silicon iron sheet or strip and is characterized by having 50 percent or more of its component grains oriented so that 4 of the cube edges of the unit cells of such'grainsare substantially parallel to the ,it is desirable to have as high 'a degree of grain orientation as is attainable, preferably more than in order that the magnetic properties in the plane of the sheet and in the rolling direction may approach the maximum attained in single crystals in the direc tion.

In actual steel mill practice, these alloys are cast into ingots, hot worked, usually by rolling, to less than 0.15 inch thick sheets or strips called band and then subjected to varying schedules of usually unidirectional cold rolling and heat treatment to produce the final, oriented sheet or strip, usually about 0.010 to 0.015 inch in thickness. Unfortunately, the degree of final grain orientation produced by prior known practices has been quite variable and has necessitated magnetic testing of samples cut from each such strip to determine its degree of orientation, since such materials having relatively low degrees of orientation are undesirable for many electrical applications and hence are not as useful as materials" having higher degrees of orientation. In previous practices it is not uncommon that oriented sheet or strip prepared from batches or heats having substantially identical compositions and processed in substantially identical fashion will yield finished strips or sheets only about 60 percent or less of which have 70 percent or more of their grains oriented as desired.

A principal object of my invention is the provision of a method of fabrication of such silicon-iron alloys to insure that the highest attainable degree of grain orientation may be consistently produced in the final sheet or strip material. Other and specifically different objects of my invention will become apparent to those skilled in the art from the following detailed disclosure.

Briefly stated, in accordance with one aspect of my invention, I have discovered that by properly controlling the grain size of such materials at an intermediate fabrication stage that the highest degree of grain orientation may be consistently produced in the final grain oriented sheet or strip material.-

My invention will be better understood from the following detailed description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing Figure 1 is a graphical representation of the variation in degree of orientation of silicon-iron with respect to intermediate grain size resulting from one method of fabrication;

Figure 2 is a graph similar to Figure 1 for a different method of fabrication; and

Figure 3 is a graph similar to Figure 1 for a still different method of fabrication.

In order to more particularly disclose my invention a series of specimens were prepared from a representative specimen of commercially produced hot rolled band 0.105 inch in thickness having the following composition, by chemical analysis: 3.22 percent by Weight silicon, 0.052 percent manganese, 0.015 percent sulfur, 0.024 percent carbon, 0.076 percent copper, 0.054 percent nickel, 0.008 percent phosphorus, 0.010 percent tin, a detectable trace of both aluminum and chromium and the balance iron.

The microstructure of this hot rolled band was of an incompletely recrystallized nature, however, it has been found that this material may be annealed to the completely recrystallized state if desired.

A portion of this hot rolled band as received was unidirectionally cold rolled to an intermediate thickness of 0.029 inch and portions of the so-rolled strip were annealed in commercial dry hydrogen (dew point about 1 60 F.), each portion for a particular time at a particular temperature. In this heat treatment individual portions of the rolled strip were placed on an iron block in the furnace at the temperatures indicated and permitted to remain on the block for the times indicated in Table I. It was found that the rolled strip required about a minute or less to attain a temperature within 5 C. of the block.

After the rolled strips were heat treated in this manner they were cooled to room temperature and a portion of each was removed and the average grain size thereof measured by the conventional lineal analysis technique.

The remainder of each strip was then cold rolled to 0.014 inch thickness and portions of each strip were decarburized by annealing in Wet hydrogen (dew point about 90 F.) at 800 C. for 5 minutes. This treatment reduced the carbon content to about 0.002%. These portions of the strips were then annealed at 1000 C. in dry hydrogen (dew point about 60 F.), the heat treatment beginning at 800 C. and reaching 1000 C. in about 20 minutes Where it was held for 3 hours.

These annealed strips were then cooled to room temperature, found to possess the desired (110) [001] texture in varying degrees and the degree of orientation or (110) [001] texture of each Was determined by means of a conventional magnetic torque test or torque magnetometer measurement as is Well known in the art. Briefly, in this test a disk-like specimen of the material is supported in a unidirectional magnetic field with the axis of the disk perpendicular to the direction of the field. The disk is then rotated in the field and the variation in the amount of torque necessary to rotate the disk about its axis is measured with respect to the angular deflection from the rolling direction. As previously stated, the optimum magnetic properties in crystals of iron and the previously described iron alloys are in the [100] direction or along the cube edges. The next easiest direction of magnetization is in the I110] direction. As this specimen is rotated in the unidirectional field in the test, it tends to align itself with a direction of easy magnetization parallel to the direction of the field and to resist movement from such a preferred alignment. By comparing the results of the test with the known torque magnetometer measurements of single crystal silicon-iron having the 110) [001] orientation and of substantially the same composition, the amount of material in any given specimen which is oriented in the desired texture may be determined and expressed in terms of percentage. The measured intermediate grain size of each of the specimen strips for each intermediate heat treatment and the degree of orientation expressed as percent. (110) [0011 texture is listed in Table I. I

Table I Intermediate anneal Grain size (110) [001] Specimen (mm.) texture Temp. C.) Time (percent) (min) 1 Not annealed. 8 2. 700 5 0.0090 17 3 15 .0118 27 4. 00 .0119 2 5. 1,020 .0157 37 6... 5 .0142 68 7 15 .0160 83 8- 60 .0170 86 9 5 .0160 85 10 15 .0167 82 11 00. .0220 88 12 5 .0180 85 13. 15 .0200 88 14.. 16 0258 82 l5 5 0153 78 16.. 15 .0228 66 17 .0400 40 18 5 .0240 19 5 .0320 40 20 10 .0330 45 21 15 .0470 39 00 .0730 42 The relat1onsh1p between the lntermediate measured gram size expressed as average gram diameter in m1ll1- of the intermediate anneal are controlling factors which determine the intermediate grain size and that the intermediate grain size has a direct and controlling relationship to the degree of grain orientation attainable. in the final anneal strip or sheet material.

Other remaining portions of the cold rolled 0.014 inch thick strip were decarburized as previously disclosed, but were given a final anneal at 1175 C. in dry hydrogen the annealing treatment starting at 800 C., the temperature was increased at a rate of about C. per hour until 1175 C. was reached and then held at this temperature for 3 hours. Since the intermediate annealing schedule and resulting intermediate grain size for these 0 specimens are reproduced in Table I, only the identifylisted.

Table II [001] texture Specimen (percent) From the foregoing data, graphically represented in Figure 2 in a manner similar to that in Figure 1, it will be readily apparent that again the intermediate grain size controls the degree of grain orientation attainable in the final strip or sheet. It will also be seen by comparing the data in Tables I and II and Figures 1 and 2 that in general the same range of intermediate grain'sizes produces a maximum .degree of (110) [001] texture for both final heat treatments, 1. e. an average measured grain diameter of from about 0.01 to about 0.03 mm.

Another portion of this same hot rolled band 0.105

inch in thickness as previously referred to in conjunction with the procedures previously outlined relating to the data presented in Tables I and II was unidirectionally cold rolled to 0.028 inch intermediate thickness. Strip specimens were prepared from this cold rolled material and annealed under 13 different temperature and time conditions in dry hydrogen (dew point 60 F.) as set forth in Table III. The average grain size of each of these annealed specimens were measured using the conventional lineal analysis technique and are listed in Table III.

Each of these strip specimens were then unidirectionally cold rolled to a final thickness of 0.014 inch thick and treated in the following manner. The specimens were decarburized by heating at 800 C. for 5 minutes in a conventional combusted gas atmosphere prepared by burning a mixture comprising about 6.5 to 1 air-to-gas ratio in a conventional atmosphere-gas converter. This atmosphere contained approximately 5 percent CO percent CO, 14 percent H 1.5 percent CH and 69.5 percent N and had a dew point of about 90 F. It should be noted that other well known dec-ar-. burizing atmospheres may be substituted for this particular atmosphere within the skill of the art.

The decarburized specimens were then enclosed in a welded metal box, a dry hydrogen (dew point about 60 F.) atmosphere was provided the interior of the box and the box was charged into a furnace at 200 C. The furnace temperature was raised to 1175 C. as measured at the furnace roof in 4 hours, held at that temperature for 8 hours and furnace cooled to 200 C. in 32 hours. The specimens were removed from the annealing box and after they had cooled to room temperature their degree of orientation was determined by torque magnetometer measurements and are shown in Table III and plotted in Figure 3 in a manner similar to that of Figures 1 and 2.

Table 111 Intermediate anneal (110) Specimen Grain size [001] (mm.) texture em 'Iirne (p 0.) (min) It is apparent from the foregoing data that again the intermediate grain size is critical in attaining the highest degree of grain orientation in a final sheet or strip material and further that in all cases this optimum intermediate grain size lies between about 0.01mm. to about 0.03 mm. and preferably between about 0.012 to 0.027

' ment that the decarburization It will be seen that these desirable intermediate grain sizes may be attained by unidirectionally cold rolling an incompletely or completely recrystallized body of this material such as for example, hot rolled band, to effect at least a 40 percent cold reduction, annealing at a temperature of between 700 to less than 1000 C. for a length of time sufiicient to produce an average measured grain size of from about 0.01 mm. to about 0.03 mm., cold reducing the annealed material at least 40 percent by unidirectional rolling, decarburizing and annealing the cold worked material at a temperature of from about 1000 to 1200 C. for a time sufficient to develop the desired high degree of [001] texture.

As will be apparent to those skilled in the art that while the specific examples previously disclosed all include a decarburization step preceding the final annealing treatmay be equally well accomplished after the final anneal and further, it has been found that high degrees of this texture may be produced vithout a decarburizing treatment.

From the foregoing, it will be apparent to those skilled in the metallurgical arts that many variations and departures may be made from the details of the processing steps disclosed in the particular examples previously set forth within the scope of my invention. I therefore do not intend my invention to be limited in any manner except as defined in the appended claims.

WhatI claim as new and desire to secure by Letters Patent of the United States is:

1. The method of fabricating polycrystalline sheet-like bodies of metal consisting of electrical grade silicon-iron alloy having from about 2.5% to 4.0% silicon, comprising the steps of cold reducing an at least partially recrystallized body of such material at least 40 percent by unidirectional rolling to form a body of intermediate thickness, heat treating said cold reduced body of intermediate thickness at a temperature of from 700 C. to 1000 C. to produce a measured average grain size of from about 0.010 mm. to about 0.030 mm, cold reducing said annealed body at least 40 percent by unidirectional rolling to produce a sheet-like body of final thickness and raising the temperature of said cold worked sheet-like body to from about 950 C. to about 1200 C. for a time sufficient to develop the desired high degree of (110) [001] texture.

2. The process as set forth in claim 1 in which said partially recrystallized body comprises commercial hot rolled band. i

3. The process as set forth in claim 2 in which said heat treatment of the cold reduced body of intermediate thickness is accomplished at a temperature between 800 C. and 950 C. to produce'an average measured grain size of from about 0.012 mm. to about 0.027 mm.

4. The process as set forth in claim 1 in which the cold worked sheet-like body of final thickness is annealed at 700 C. to 900 C. in a decarburizing atmosphere to reduce the carbon content to less than 0.010 percent by weight.

References Cited in the file of this patent 

1. THE METHOD OF FABRICATING POLYCRYSTALLINE SHEET-LIKE BODIES OF METAL CONSISTING OF ELECTRICAL GRADE SILICON-IRON ALLOY HAVING FROM ABOUT 2.5% TO 4.0% SILICON, COMPRISING THE STEPS OF COLD REDUCING AN AT LEAST PARTIALLY RECRYSTALLIZED BODY OF SUCH MATERIAL AT LEAST 40 PERCENT BY UNIDIRECTIONAL ROLLING TO FORM A BODY INTERMEDIATE THICKNESS, HEAT TREATING SAID COLD REDUCED BODY OF INTERMEDIATE THICKNESS AT A TEMPERATURE OF FROM 700*C. TO 1000*C. TO PRODUCE A MEASURED AVERAGE GRAIN SIZE OF FROM ABOUT 0.010 MM. TO ABOUT 0.030 MM., COLD REDUCING SAID ANNEALED BODY AT LEAST 40 PERCENT BY UNIDIRECTIONAL ROLLING TO PRODUCE A SHEET-LIKE BODY OF FINAL THICKNESS AND RAISING THE TEMPERATURE OF SAID COLD WORKED SHEET-LIKE BODY TO FROM ABOUT 950*C. TO ABOUT 1200*C. FOR A TIME SUFFICIENT TO DEVELOP THE DESIRED HIGH DEGREE OF (110) (001) TEXTURE. 